Thallium(I) Tropolonates: Synthesis, Structure, Spectral Characteristics, and Antimicrobial Activity Compared to Lead(II) and Bismuth(III) Analogues

Synthesis, single-crystal X-ray determination diffraction and FT-IR, NMR (1H, 13C, 19F and 205Tl), UV–vis, and luminescence spectra characteristics were described for series of thallium(I) compounds: thallium(I) triflate (Tl(OTf)), 1:1 co-crystals of thallium(I) triflate and tropolone (Htrop), Tl(OTf)·Htrop, as well as simple thallium(I) chelates: Tl(trop) (1), Tl(5-metrop) (2), Tl(hino) (3), with Htrop, 5-methyltropolone (5-meHtrop), 4-isopropyltropolone (hinokitiol, Hhino), respectively, and additionally more complex {Tl@[Tl(hino)]6}(OTf) (4) compound. Comparison of their antimicrobial activity with selected lead(II) and bismuth(III) analogs and free ligands showed that only bismuth(III) complexes demonstrated significant antimicrobial activity, from two- to fivefold larger than the free ligands.

In the Cambridge Structural Database, there are several crystal structures that include thallium(I) triflate as a part of the structure [ The molecular structures of the studied complexes with tropolone and its derivative substituted in position 5 by the methyl group are presented in Figure 3, and their selected bond parameters are listed in Table 2. The asymmetric units of the presented structures consisted of either a half or one molecule of the thallium(I) complex in 1 or 2, respectively. The crystal structures of these two compounds consisted of mononuclear Tl(trop) or Tl(5metrop) units in which tropolones behaved as anionic bidentate ligands. Each thallium(I) ion was chelated by one ligand, resulting in a mean Tl-O bond length of 2.472(9) Å in 2 and 2.588(3) Å in 1. The bite angles, formed within five-membered chelate rings, equaled 61.1(1) and 65.7(3)° for 1 and 2, respectively. The C-O bonds in the range of 1.28-1.30 Å were distinguished within each ligand moiety. Thus, these distances were intermediate between those observed for double C=O (ca. 1.25 Å) and single C-O (ca. 1.34 Å) bonds in the free tropolone and 5-methyltropolone molecules [38,52]. In the Tl(OTf)·Htrop structure, the tropolone molecules were not involved in the formation of O-H···O hydrogen-bonded dimers, as they were in the case of pure tropolone [38]. However, it was stabilized by intermolecular O2-H2···O4 hydrogen bonds of 2.941(3) Å formed between tropolone molecules and triflate anions.
2.2.2. Complexes with Tropolone and 5-Methyltropolone: Tl(trop) (1) and Tl(5-metrop) (2) The molecular structures of the studied complexes with tropolone and its derivative substituted in position 5 by the methyl group are presented in Figure 3, and their selected bond parameters are listed in Table 2. The asymmetric units of the presented structures consisted of either a half or one molecule of the thallium(I) complex in 1 or 2, respectively. The crystal structures of these two compounds consisted of mononuclear Tl(trop) or Tl Coordination environments around chelated thallium(I) ions in the studied complexes, as shown in Figure 4, were completed by three or four additional oxygen atoms from neighboring molecules. These thallium-oxygen contacts were much shorter than the calculated sum of their van der Waals radii (3.48 Å [37]). The crystal structure of 1 was stabilized by a connection of the Tl(trop) units into layers ( Figure 5), with two types of bridging Tl···O distances of 2.906(3) and 2.947(3) Å between adjacent molecules. The aromatic rings were directed outside of these layers. The molecules of 1 could also be considered as placed in planes intersecting at an angle of approximately 86° with distances of 2.51 Å between them.
Coordination environments around chelated thallium(I) ions in the studied complexes, as shown in Figure 4, were completed by three or four additional oxygen atoms from neighboring molecules. These thallium-oxygen contacts were much shorter than the calculated sum of their van der Waals radii (3.48 Å [37]). The crystal structure of 1 was stabilized by a connection of the Tl(trop) units into layers (   Table 2). Solid and dashed lines indicate chelating bonds and intermolecular contacts, respectively. The crystal structure of 2 could be described as a connection of chelate units into chains extending along the b-axis ( Figure 6). Within two adjacent chains the molecules of 2 were held together by bridging thallium-oxygen contacts of 2.873(10), 2.888(10), and 3.189(9) Å, clearly longer than those observed for the chelating bonds. In addition, these pairs of chains were stabilized together by the presence of weak intermolecular hydrogen bonds of 3.35(1) (C4-H4···O1), 3.90(1) (C8-H8C···C6), and 4.01(2) Å (C8-H8C···C7). Moreover, in contrast to 1, the crystal structure of the complex with 5-methyltropolonato ligand was stabilized through intermolecular π···π interactions between neighboring aromatic rings, with distances of 3.84 Å between their centroids. In addition, the shortest distance between metal atoms in 2 (3.843(2) Å) was a bit less than twice the van der Waals radius of thallium (3.92 Å [37]), and could be considered as a weak metallophilic bonding. This distance was shorter than the analogous contacts in 1 (4.1822(2) and 4.1926(2) Å). It was noteworthy that in contrast to 1, the complex with 5-methyltropolone had a nonplanar chelate ring with the Tl atom lying about 0.38 Å out of plane formed by the remaining atoms. It is worth adding that only a few complexes of 5-methyltropolone with bismuth(III) ion have been structurally characterized until now [15].  Table 2). Solid and dashed lines indicate chelating bonds and intermolecular contacts, respectively.   Table 2). Solid and dashed lines indicate chelating bonds and intermolecular contacts, respectively. The crystal structure of 2 could be described as a connection of chelate units into chains extending along the b-axis ( Figure 6). Within two adjacent chains the molecules of 2 were held together by bridging thallium-oxygen contacts of 2.873(10), 2.888(10), and 3.189(9) Å, clearly longer than those observed for the chelating bonds. In addition, these pairs of chains were stabilized together by the presence of weak intermolecular hydrogen bonds of 3.35(1) (C4-H4···O1), 3.90(1) (C8-H8C···C6), and 4.01(2) Å (C8-H8C···C7). Moreover, in contrast to 1, the crystal structure of the complex with 5-methyltropolonato ligand was stabilized through intermolecular π···π interactions between neighboring aromatic rings, with distances of 3.84 Å between their centroids. In addition, the shortest distance between metal atoms in 2 (3.843(2) Å) was a bit less than twice the van der Waals radius of thallium (3.92 Å [37]), and could be considered as a weak metallophilic bonding. This distance was shorter than the analogous contacts in 1 (4.1822(2) and 4.1926(2) Å). It was noteworthy that in contrast to 1, the complex with 5-methyltropolone had a nonplanar chelate ring with the Tl atom lying about 0.38 Å out of plane formed by the remaining atoms. It is worth adding that only a few complexes of 5-methyltropolone with bismuth(III) ion have been structurally characterized until now [15]. The crystal structure of 2 could be described as a connection of chelate units into chains extending along the b-axis ( Figure 6). Within two adjacent chains the molecules of 2 were held together by bridging thallium-oxygen contacts of 2.873(10), 2.888(10), and 3.189(9) Å, clearly longer than those observed for the chelating bonds. In addition, these pairs of chains were stabilized together by the presence of weak intermolecular hydrogen bonds of 3.35(1) (C4-H4···O1), 3.90(1) (C8-H8C···C6), and 4.01(2) Å (C8-H8C···C7). Moreover, in contrast to 1, the crystal structure of the complex with 5-methyltropolonato ligand was stabilized through intermolecular π···π interactions between neighboring aromatic rings, with distances of 3.84 Å between their centroids. In addition, the shortest distance between metal atoms in 2 (3.843(2) Å) was a bit less than twice the van der Waals radius of thallium (3.92 Å [37]), and could be considered as a weak metallophilic bonding. This distance was shorter than the analogous contacts in 1 (4.1822(2) and 4.1926(2) Å). It was noteworthy that in contrast to 1, the complex with 5-methyltropolone had a nonplanar chelate ring with the Tl atom lying about 0.38 Å out of plane formed by the remaining atoms. It is worth adding that only a few complexes of 5-methyltropolone with bismuth(III) ion have been structurally characterized until now [15].

Complexes with Hinokitiol: Tl(hino) (3) and {Tl@[Tl(hino)] 6 }(OTf) (4)
The crystal structure of 3 contained four Tl(hino) molecules in the asymmetric unit (Figure 7), while the asymmetric part of 4 consisted of six such chelate molecules and one thallium(I) cation neutralized by a triflate anion (Figure 8). This Tl + ion (Tl7) was isolated from the CF 3 SO 3 − ion by six Tl(hino) molecules, which formed a specific cage for the free metal cation. The crystal structure of 3 contained four Tl(hino) molecules in the asymmetric unit (Figure 7), while the asymmetric part of 4 consisted of six such chelate molecules and one thallium(I) cation neutralized by a triflate anion ( Figure 8). This Tl + ion (Tl7) was isolated from the CF3SO3 − ion by six Tl(hino) molecules, which formed a specific cage for the free metal cation.
In the presented compounds with a hinokitiol ligand, some part of the thallium(I) ions, similarly to 2, remained clearly outside the plane of the chelate ring, with a displacement value of approximately 0.59 and 0.64 Å for the Tl4 and Tl3 atoms in 3 and 0.27 and 0.54 Å for the Tl5 and Tl3 atoms in 4.       Table  3) and (b) 4 (symmetry codes as in Table 4). Solid and dashed lines indicate chelating bonds and intermolecular contacts, respectively.  In the presented compounds with a hinokitiol ligand, some part of the thallium(I) ions, similarly to 2, remained clearly outside the plane of the chelate ring, with a displacement value of approximately 0.59 and 0.64 Å for the Tl4 and Tl3 atoms in 3 and 0.27 and 0.54 Å for the Tl5 and Tl3 atoms in 4.
All Tl(hino) complexes in the extended crystal structures were stabilized by weaker thallium-oxygen contacts between neighboring molecules. These additional interactions are listed in Tables 3 and 4. Most of such bridging Tl···O distances in 3 and 4 remained in the range of 2.72-3.08 Å, much shorter than the 3.48 Å calculated for the sum of the van der Waals radii of both atoms [37]. A few longer contacts of approximately 3.41, 3.43, and 3.45 Å could also be found in 4. In addition, some intermolecular anagostic Tl···H-C interactions of about 2.92 (Tl6···H47), 2.93 (Tl1···H57), and 2.99 Å (Tl5···H37), shorter than the accepted sum of the van der Waals radii of Tl and H atoms (3.06 Å [37]), appeared to contribute to the stability of the crystal packing in 4. The corresponding Tl-H-C angles associated with these contacts were 123 (for Tl6), 133 (Tl1), and 122 • (Tl5). These anagostic Tl···H-C interactions found in 4 were shorter than those for other thallium(I) systems presented recently [53,54].
The position of the free Tl + ion in 4 was stabilized by six thallium-oxygen contacts with a mean Tl···O distance of 2.906(6) Å and a narrow distance distribution (range: 2.868-2.979(6) Å) showing a distorted octahedral configuration around this metal center. In the crystal structures of both compounds, the Tl(hino) molecules were arranged into layers ( Figure 10) with isopropyl groups directed outside of them. In addition, the triflate anions were placed between these layers in 4.

Spectroscopic Analysis
Comparison of the IR spectrum of the Tl(OTf)·Htrop adduct with the spectra of pure tropolone and thallium(I) triflate components (Figures S1-S3) allowed us to select two groups of the IR bands corresponding to (i) tropolone and to (ii) triflate anion vibrations. The former dominated the spectra of the described system. The broad band at 3170 cm −1 could be assigned to the tropolone OH stretching vibrations. The bending and torsional OH modes could be distinguished at 1254 and 759 cm −1 [56]. The asymmetric and symmetric stretching tropolone C=O bands could be found at 1607 and 1532 cm −1 . The main IR bands of the triflate ion were those of the SO 3 and CF 3 group vibrations located at 1218, 1154, and 1021 cm −1 .
The juxtaposition of the IR spectra of tropolone ligands with the reciprocal thallium(I) complexes ( Figures S3-S9) showed that the tropolonate bands dominated the spectra. The thallium(I) chelates were formed through the tropolonate C=O and O-H moieties, which were corroborated by the absence of the stretching and bending OH bands at 3187 and 1264, 3190 and 1261, or 3203 and 1268 cm −1 in free tropolone, 5-methyltropolone, or hinokitiol, respectively. This was accompanied by a decrease in the double bond character of the C=O group coordinated to the metal ion, manifested by a downshift of the C=O bands at 1603 and 1543 (Htrop) or 1619 and 1542 cm −1 (5-meHtrop), respectively, to 1588 and 1492 cm −1 or 1605 and 1508 cm −1 in the thallium(I) complexes. The analogous hinokitiol bands at 1606 and 1541 cm −1 shifted to 1582 and 1490 cm −1 in the thallium(I) hinokitiolate complexes (3 and 4).
The UV-vis spectra of tropolone ligands and the 1-4 thallium(I) complexes in the methanol solutions were similar to each other ( Figure S10). Indeed, as the free ligands, 1-4 exhibited intense absorptions below 250 nm and two distinct bands in the 300-420 nm region: more intense at 332 nm and weaker at 395 nm. The additional band could be seen between them as a shoulder at ca. 370 nm. In this very region, the bands of complexes were redshifted and more distinct than those of the pure ligands.
The photophysical data of thallium(I) chelates studied in methanol and the solid state are summarized in Table 5, while the emission and absorption profiles can be found in Figure S11. The fluorescence spectra of all compounds in methanol solution showed very broad profiles, from around 380 up to 550 nm (left panels of Figure S11). In the solid state, two bands around 420 and 462 nm (emission spectra) and 375 and 420 nm (excitation spectra) were observed (right panels of Figure S11). Only 2 exhibited fluorescence that was redshifted (502 nm) in comparison to the other thallium(I) chelates. The fluorescence decays had two components in the solid state: a long-lived one, with a lifetime around 15 ns; and the other short-lived, with a lifetime around 0.15 ns. In the solid-state measurements, the highest average lifetime was found for 4, and the shortest for 1. Interestingly, in the methanol solution, this trend was reversed. In methanol, the fluorescence quantum yields ϕ of the thallium(I) chelates were relatively high. Indeed, the complexes with tropolone and 5-methyltropolone exhibited 3 or 4 times higher quantum yields than for complexes with hinokitiol. Furthermore, all the studied complexes displayed very short-lived emissions τ if compared to, e.g., thallium(I) acetyloacetonate [57]. The photophysical data of the two hinokitiol complexes (3 and 4) were almost the same. The NMR data of all compounds in solution are gathered in Figures S12-S17. The 1 H NMR spectra of thallium(I) compounds showed three (Tl(OTf)·Htrop and 1), five (2), or six (3 and 4) proton signals originating from the respective ligand. In the case of the adduct, the signal of the OH proton in Htrop was not registered. However, for the [Bi(trop) 2 (Htrop)(CF 3 SO 3 )] and [Pb(trop) 2 (Htrop)] complexes, a broad proton signal of the tropolone OH group was previously observed by us at around 10 ppm [16]. In the 205 Tl NMR spectra, the Tl + signal from the thallium(I) triflate and its adduct with tropolone was located at 370 ppm ( Figure S18). Complexation by tropolonates broadened the signal and shifted it to the lower field by over 1000 ppm . For 4, 1, 3, and 2, the 205 Tl NMR signals were observed at 1373, 1416, 1507, and 1574 ppm, respectively ( Figure S18). Surprisingly, the 205 Tl NMR signals of the Tl(hino) chelates 3 and 4 differed by about 130 ppm. Due to the relatively low concentration of the uncoordinated thallium(I) ion in 4, its signal could not be registered.

Antimicrobial Activity
The in vitro antimicrobial activity of thallium(I) tropolonate complexes and free tropolone ligands was compared with the selected lead(II) and bismuth(III) analogues characterized chemically earlier [14,15,17]. The activity of three thallium(I) (Tl(trop), Tl(5-metrop) and Tl(hino)), two lead(II) (Pb(hino) 2 and Pb(trop) 2 ), and three bismuth(III) ([Bi 2 (trop) 4 (NO 3 ) 2 ] n , Bi(trop) 3 and Bi(5-metrop) 2 (NO 3 )) complexes and their ligands against two groups of reference microorganisms were examined. The first one was composed of strains recommended by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) for the antibiotics quality control [58]. This group contained microorganisms representative of the most frequent infections in humans: Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 29213, and Enterobacter faecalis ATCC 29212. The second group consisted of microorganisms indicated by the United States Pharmacopeia to test the antimicrobial effectiveness [59]. It contained the following strains: E. coli ATCC 8739, P. aeruginosa ATCC 9027, S. aureus ATCC 6538, and Candida albicans ATCC 10231. The minimum inhibitory concentration (MIC) values for the two groups of bacterial species were compared with the results included in EUCAST breakpoint tables for interpretation of MICs (Table 6). Surprisingly, none of the thallium(I) complexes displayed potent antimicrobial activity. Moreover, the free 5-meHtrop ligand had high activity against E. faecalis ATCC 29212 (MIC = 2 mg·mL −1 ), whereas the activity of the Tl(5-metrop) complex was significantly reduced (MIC = 256 mg·mL −1 ). This suggested that the α-hydroxycarbonyl moiety with neighboring OH and C=O groups, present in the free ligand and blocked in the thallium(I) complex, played a crucial role in the antimicrobial activity of the free 5-meHtrop ligand. In addition, the two lead(II) complexes showed no significant activity against the tested microorganisms. In contrast, but in line with previous reports [60][61][62], the bismuth(III) complexes demonstrated significant antimicrobial activity. It was found earlier that some bismuth(III) tropolonates showed higher activity against Helicobacter pylori in the agar dilution test than the standard bismuth(III) salicylate [13]. Nevertheless, the bismuth(III) hinokitiolate complex did not demonstrate a specific antimicrobial action [7]. The mechanism of the bismuth compounds antimicrobial activity is not fully understood. Yet, it is known that they increase the permeability of the bacterial cell membrane and reduce the glycocalyx production [63,64]. All tested bismuth(III) complexes showed 2-to 5-fold larger activity than their free ligands (Table 6). In the case of S. aureus ATCC 29213, the MIC values were 8 mg·mL −1 for the bismuth(III) complexes, and 128 mg·mL −1 for ligands. For this very reference strain, the MIC cut-off values of the majority of the antibiotics administrated for treatment were significantly lower. However, for chloramphenicol, an antibiotic used to treat staphylococci infections, the breakpoint was 8 mg·mL −1 as well. The same MIC value was also demonstrated by the Bi(trop) 3 complex against E. coli ATCC 25922 and E. faecalis ATCC 29212, and also by the Bi(5-metrop) 2 (NO 3 ) complex against S. aureus ATCC 6538, E. coli ATCC 25922, and P. aeruginosa ATCC 9027. On the other hand, for amikacin, a broad-spectrum antibiotic to treat infections caused by Gram-negative and Gram-positive bacteria, the breakpoint against P. aeruginosa was 16 mg·mL −1 . Thus, the Bi(5-metrop) 2 (NO 3 ) complex displayed a two-times lower MIC.
Much less satisfactory were the minimum bactericidal concentration values; MBC = 16 mg·mL −1 was obtained for [Bi 2 (trop) 4 (NO 3 ) 2 ] n and Bi(trop) 3 complexes against E. coli ATCC 25922, S. aureus ATCC 29213, and S. aureus ATCC 6538. Moreover, the two complexes exhibited MBC = 64 mg·mL −1 against P. aeruginosa ATCC 27853 and P. aeruginosa ATCC 9027. The Bi(trop) 3 complex showed a killing activity of 256 mg·mL −1 with MBC for E. coli ATCC 8739. However, the MBC for the Tl(hino) complex was equal to 512 mg·mL −1 against both: E. coli strains, ATCC 25922, ATCC 8739, and E. faecalis ATCC 29212, but the other thallium(I) complexes did not show a killing effect in the tested range of concentrations (Table 6).

Materials and Methods
The 5-Methyltropolone was synthesized as presented earlier [52]. All the other reagents were used as purchased from commercial sources. The IR spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the 4000-650 cm −1 range with 4 cm −1 resolution using a PIKE Technologies MIRacle (Madison, WI, USA) accessory equipped with ZnSe crystal designed for single reflection horizontal ATR technique. UV-vis spectra were registered with Thermo Scientific Evolution 600 (Thermo Fisher Scientific, Waltham, MA, USA) or Perkin-Elmer Lambda 35 spectrometers (Waltham, MA, USA) using methanol solutions of the studied complexes and ligands. Emission spectra and fluorescence lifetimes at room temperature were collected on a Horiba Fluorolog 3 fluorimeter (Horiba Scientific, Piscataway, NJ, USA). Excitation for fluorescence lifetime measurements was provided by Delta Diode 336 and 303 nm for solutions and solid states, respectively. The 1 H, 13 C, 19 F, and 205 Tl NMR spectra of DMSO-d 6 solutions were recorded at 298 K on a Bruker AVANCE III HD 500 MHz spectrometer (Bruker, Billerica, MA, USA). An aqueous solution of thallium(I) nitrate (1 mol·dm −3 ) was used as a chemical shift reference (0 ppm). The numbering of carbon atoms in the description of NMR signals is shown in Scheme 1.

Scheme 1.
Atom labeling in tropolonato-like ligands used for NMR assignments.

Preparation of the Thallium(I) Compounds
3.1.1. Tl(CF3SO3) (Tl(OTf)) The thallium(I) triflate was synthesized in the reaction of metallic thallium with concentrated aqueous solution of trifluoromethanesulfonic acid, followed by evaporation of the excess of water and acid. ATR-IR (ZnSe), νmax/cm −1 : 1217 vs (SO3), 1160 m (CF3), 1015 m (SO3), 763 w (CF3); assignments based on [65]. 13  Thallium(I) nitrate (0.100 g, 0.375 mmol) was dissolved in hot water (1.0 mL), and to this solution, tropolone (0.045 g, 0.377 mmol) in methanol (0.5 mL) was added. To the resulting mixture, an aqueous ammonia solution (28 µL, 25%, 0.376 mmol NH3) dissolved in methanol (0.2 mL) was slowly added dropwise. The whole mixture was stirred at all times. During the addition of ammonia solution, the mixture had already turned yellow, and a yellow crystalline material of 1 began to precipitate. After a few hours, this compound was isolated from the solution. Yield: 0.058 g, 47  The thallium(I) triflate was synthesized in the reaction of metallic thallium with concentrated aqueous solution of trifluoromethanesulfonic acid, followed by evaporation of the excess of water and acid. ATR-IR (ZnSe), ν max /cm −1 : 1217 vs (SO 3 ), 1160 m (CF 3 ), 1015 m (SO 3 ), 763 w (CF 3 ); assignments based on [65]. 13  Thallium(I) nitrate (0.100 g, 0.375 mmol) was dissolved in hot water (1.0 mL), and to this solution, tropolone (0.045 g, 0.377 mmol) in methanol (0.5 mL) was added. To the resulting mixture, an aqueous ammonia solution (28 µL, 25%, 0.376 mmol NH 3 ) dissolved in methanol (0.2 mL) was slowly added dropwise. The whole mixture was stirred at all times. During the addition of ammonia solution, the mixture had already turned yellow, and a yellow crystalline material of 1 began to precipitate. After a few hours, this compound was isolated from the solution. Yield: 0.058 g, 47.5% (based on Tl). Anal. Calc.

Single-Crystal X-ray Diffraction
X-ray diffraction data for the obtained crystals were collected at 100 K on a Rigaku SuperNova (dual source) diffractometer equipped with an Eos CCD detector using the mirror-monochromated Mo Kα or Cu Kα radiations (λ = 0.71073 or 1.54184 Å) from a microfocus Mova or Nova X-ray sources, respectively. A suitable single crystal was mounted on a nylon loop using cryoprotectant oil (Paratone-N). The majority of the solid Tl(OTf) compound was noncrystalline, giving no reflections in the course of pre-experiment during the X-ray analysis. However, it succeeded in finding the more ordered material suitable for scXRD measurement. Crystallographic parameters and refinement details for all studied compounds are given in Table S1. Data collection and reduction were performed using CrysAlis PRO software. For the crystals, the empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was introduced. The crystal structures were solved by the direct method and refined by the full-matrix least-squares method on F 2 data. All nonhydrogen atoms (except for O3, O4, O5, O6, and C1 atoms in Tl(OTf)) were refined with anisotropic atomic displacement parameters. All H atoms bonded to C atoms were placed in calculated positions with C-H = 0.95 (aromatic), 0.98 (methyl), or 1.00 Å (methine) and refined isotropically using a riding model with U iso (H) = 1.5 U eq (C) for the methyl H atoms and 1.2 U eq (C) for the remaining H atoms. The H atom of the OH group in Tl(OTf)·Htrop was located in a difference map, and its position and isotropic thermal parameter U iso were freely refined. All calculations were performed with the SHELXTL program package [66] integrated with the OLEX2 crystallographic software [67]. The MERCURY program [68] was applied for the structural graphics.

Antimicrobial Activity
The in vitro antimicrobial activity of ligands and complexes dissolved in DMSO was determined according to ISO 20776-1 [69]. The minimum inhibitory concentrations (MICs) and minimum bactericidal/fungicidal concentrations (MBCs/MFCs) were tested for reference microorganisms obtained from the ATCC Culture Collection [70]: E. coli ATCC 8739, E. coli ATCC 25922, P. aeruginosa ATCC 9027, P. aeruginosa ATCC 27853, S. aureus ATCC 6538, S. aureus ATCC 29213, E. faecalis ATCC 29212, and C. albicans ATCC 10232. The procedure consisted of the preparation of 2-fold dilutions of the tested compounds in liquid growth medium dispensed into a 96-well microtiter plate. Serial dilutions were carried out from 512 to 0.5 mg·mL −1 and inoculated with standardized microbial suspension (adjusted to 0.5 McFarland scale) prepared in the same media in a total volume of 200 mL per well. Culture plates were incubated at 35 • C for 18 h. The lowest concentration of compound inhibiting visible bacterial/fungal growth was referred to as the MIC, and that causing inoculum death was reported as the MBC/MFC.

Conclusions
Thallium(I) triflate (Tl(OTf)) was synthesized in the reaction of the concentrated aqueous solution of trifluoromethanesulfonic acid with metallic thallium. Co-crystals of thallium(I) triflate and tropolone (Tl(OTf)·Htrop) were obtained from a mixture of both compounds at a 1:1 molar ratio. Reactions of tropolone (Htrop) and 5-methyltropolone (5-meHtrop) with thallium(I) triflate or nitrate(V) in methanol solution after addition of an equimolar amount of ammonia led to the formation of the simple chelates: Tl(trop) (1) and Tl(5-metrop) (2). In the case of 4-isopropyltropolone (hinokitiol, Hhino), a similar process gave the simple Tl(hino) complex (3) when using thallium(I) nitrate(V) and the more complex compound {Tl@[Tl(hino)] 6 }(OTf) (4) for the thallium(I) triflate salt. The crystal structures of thallium(I) triflate, its co-crystal with tropolone, and complexes 1-4 were determined by means of single-crystal X-ray diffraction and characterized by FT-IR, NMR ( 1 H, 13 C, 19 F, and 205 Tl), UV-vis, and luminescence techniques. Antimicrobial activity of thallium(I) complexes was tested and compared with free ligands and selected analogous lead(II) and bismuth(III) compounds. None of the thallium(I) complexes showed significant antimicrobial activity. Of the other compounds tested, only the bismuth(III) complexes presented potentially interesting activity, two to five times larger than their free ligands.